Alyssa Abbey Dissertation Defense

Assessing Rio Grande rift development and exhumation in the southern Rocky Mountains using techniques in low-temperature thermochronology

Congratulations to Alyssa Abbey who defended her dissertation on April 27, 2018

Advisor: Nathan Niemi

Abstract

I use low-temperature thermochronometry methods to assess a variety of tectonic processes including continental rifting, fault initiation and growth patterns, orogenic exhumation and reheating mechanisms. Specifically this dissertation focuses on the development of the Rio Grande rift (RGR) in New Mexico and Colorado, USA and exhumation in the southern Rocky Mountains, Colorado.To understand rift development and distinguish between different potential rifting models in a continental rift system we need to determine the spatial and temporal patterns of rift-related faulting and magmatism. Distinct phases of fault ignition, growth and linkage of basin-bounding fault systems can be documented through dense vertical transect sampling and inverse thermal history modeling, which we perform in the upper Arkansas River (UAR) Basin in the northern part of the RGR (chapter 2). Fault motion in the UAR Basin initiates on a small segment at ~25 Ma. More segments begin initiate and undergo fault growth via tip propagation for ~15 m.y. until ~10 Ma when the entire fault systems is integrated into a coherent 90-km-long fault system. We apply a similar method along the entire RGR to understand fault initiation, growth and linkage along the entire rift (chapter 3). Additionally, we evaluate spatial and temporal patterns in faulting and rift-related magmatism to obtain insight into the processes behind extension accommodation and how to differentiate between rift models. Rift initiation begins synchronously ~25 Ma on fault segments in both the northern and southern RGR. Segment initiation, growth, and linkage continues from ~25 to ~15 Ma at which time the entire rift system becomes linked through strike-slip faulting and magmatic accommodation in the central RGR. Trends in the locations of faulting and magmatism are spatially coincident with pre-existing weaknesses from previous tectonic rifting and orogenic events and crustal and lithospheric thickness differences suggesting that rift structure and geometry are at least partly controlled by both inherited structure and or lithospheric properties. Using these new analyses and interpretations we do not support a northward propagation model for the development of the RGR and instead favor a synchronous model where a commination of an oblique strain model and a block rotation form the Rio Grande rift.To constrain the magnitude and timing of Laramide deformation in the southern Rocky Mountains we combine new thermochronometric, geochronologic, and clumped isotope data from the Mosquito Range, Arkansas Hills, and Arkansas River valley (Colorado, USA) (chapter 4). Analysis of these data show that during the Laramide Orogeny ~3–5 km of differential (west side up) exhumation between the Mosquito Range–Arkansas Hills (5–7 km total exhumation from 80 and 60 Ma) and the Royal Gorge region to the east (<1–2 km exhumation since ca. 120 Ma) occurred. We also recognize an inverse trend in age elevation relationships in our thermochronometry samples and demonstrate, through the application of clumped isotopic analysis, that this inversion likely arises from post-exhumation hydrothermal reheating driven by paleotopography and overlying late Eocene to early Miocene ignimbrite sequences. We further use these data to propose that a paleo-surface often referred to as the Eocene erosion surface entirely formed in the Paleocene and suggest that the southern Rockies may be a useful region to study the evolution of paleo-landscapes. To understand rift development and distinguish between different potential rifting models in a continental rift system we need to determine the spatial and temporal patterns of rift-related faulting and magmatism. Distinct phases of fault ignition, growth and linkage of basin-bounding fault systems can be documented through dense vertical transect sampling and inverse thermal history modeling, which we perform in the upper Arkansas River (UAR) Basin in the northern part of the RGR (chapter 2). Fault motion in the UAR Basin initiates on a small segment at ~25 Ma. More segments begin initiate and undergo fault growth via tip propagation for ~15 m.y. until ~10 Ma when the entire fault systems is integrated into a coherent 90-km-long fault system.We apply a similar method along the entire RGR to understand fault initiation, growth and linkage along the entire rift (chapter 3). Additionally, we evaluate spatial and temporal patterns in faulting and rift-related magmatism to obtain insight into the processes behind extension accommodation and how to differentiate between rift models. Rift initiation begins synchronously ~25 Ma on fault segments in both the northern and southern RGR. Segment initiation, growth, and linkage continues from ~25 to ~15 Ma at which time the entire rift system becomes linked through strike-slip faulting and magmatic accommodation in the central RGR. Trends in the locations of faulting and magmatism are spatially coincident with pre-existing weaknesses from previous tectonic rifting and orogenic events and crustal and lithospheric thickness differences suggesting that rift structure and geometry are at least partly controlled by both inherited structure and or lithospheric properties. Using these new analyses and interpretations we do not support a northward propagation model for the development of the RGR and instead favor a synchronous model where a commination of an oblique strain model and a block rotation form the Rio Grande rift.To constrain the magnitude and timing of Laramide deformation in the southern Rocky Mountains we combine new thermochronometric, geochronologic, and clumped isotope data from the Mosquito Range, Arkansas Hills, and Arkansas River valley (Colorado, USA) (chapter 4). Analysis of these data show that during the Laramide Orogeny ~3–5 km of differential (west side up) exhumation between the Mosquito Range–Arkansas Hills (5–7 km total exhumation from 80 and 60 Ma) and the Royal Gorge region to the east (<1–2 km exhumation since ca. 120 Ma) occurred. We also recognize an inverse trend in age elevation relationships in our thermochronometry samples and demonstrate, through the application of clumped isotopic analysis, that this inversion likely arises from post-exhumation hydrothermal reheating driven by paleotopography and overlying late Eocene to early Miocene ignimbrite sequences. We further use these data to propose that a paleo-surface often referred to as the Eocene erosion surface entirely formed in the Paleocene and suggest that the southern Rockies may be a useful region to study the evolution of paleo-landscapes.